The present invention relates generally to a system and method for controlling the air/fuel ratio in an internal combustion engine, and, more particularly, to a system and method for controlling the air/fuel ratio in an internal combustion engine using feedback from at least one exhaust gas oxygen sensor positioned in the exhaust stream from the engine.
To minimize undesirable emissions, such as NOx, HC, and CO2, modern automotive vehicles typically include an emission control device coupled to the engine of the vehicle. For example, many vehicles are equipped with a three-way catalytic converter, which includes a catalyst material capable of storing NOx during periods when the engine is operated in a lean state, and releasing and reducing the stored NOx during periods when the engine is operated in a rich state. Other emission control devices may operate in various ways and have various objectives. In any event, most emission control devices are employed in connection with an engine air/fuel ratio control strategy that monitors and adjusts the air/fuel ratio provided to the engine in order to optimize the emission reduction capability of the emission control device.
To that end, it is known to control the engine air/fuel ratio based on feedback from one or more exhaust gas oxygen sensors positioned in the exhaust stream from the engine. For example, it is known to position an exhaust gas oxygen sensor downstream of the emission control device for the purpose of monitoring the oxygen content of the exhaust gas in the tail pipe. The output signal from the exhaust gas oxygen sensor is compared to a set point reference value to calculate an error value. The error value is generally indicative of whether the air/fuel ratio at the point of the exhaust gas oxygen sensor is rich or lean. An electronic engine controller adjusts an amount of fuel provided to the engine cylinders, and thus the air/fuel ratio therein, based at least in part on the error value. The set point reference value can be either a pre-determined constant value, or it can be determined dynamically based on one or more engine operating parameters, such as engine speed and/or load. According to either method, the set point reference value remains constant for a constant engine speed and/or load.
The inventor has recognized that having a constant set point reference value for an extended period of time tends to lead to an oxygen rich or oxygen lean condition in the catalyst, either of which tending to compromise the efficiency of the emission control device. For example, in a three-way catalytic converter, oxygen saturation of the catalyst may generate higher NOx emissions, and oxygen depletion in the catalyst may generate higher HC and CO2 emissions. Whether the set point reference value is a pre-determined constant or dynamically-determined based on engine operating parameters, the set point reference value is constant for extended lengths of time during periods of constant engine speed and/or load. Accordingly, the inventor has recognized a need for a new method and system of adjusting the engine air/fuel ratio based on an output signal of an exhaust gas oxygen sensor.
The present invention relates to a new method and system for controlling the air/fuel ratio in an engine based on the output of an exhaust gas oxygen sensor positioned in the exhaust stream from the engine. In particular, an emission control device is coupled to an internal combustion engine. An exhaust gas oxygen sensor is also positioned in the exhaust stream, preferably downstream of the emission control device. An electronic engine controller compares an output signal from the exhaust gas oxygen sensor to a set point reference value to calculate an error value. The error value is used to adjust the amount of fuel provided to the engine.
To avoid the condition where the set point reference value is constant over an extended period of time, the present invention causes the set point reference value to vary as a function of time. In various preferred embodiments of the invention, the set point reference value is derived from a periodic waveform, such as a sine waveform, a triangle waveform, or a square waveform for example, that oscillates around an average set point. Accordingly, the set point reference value always varies over time, and, even during periods of extended steady state engine operation (i.e., constant engine speed and/or load), the set point reference value is not held constant. As a result, the engine air/fuel ratio is varied during steady state engine operation, causing oxygen and reductants (HC and CO2) to migrate through the catalyst system, thus periodically refreshing the catalyst storage sites and increasing the efficiency of the emission control device.